Diesel engine exhaust oxidizer

ABSTRACT

An oxidizing device with a microprocessor control means for removing soot and other particulates from the exhaust gases of a diesel engine having an enclosure with a forward inlet for receiving diesel engine exhaust, a main flow path through the enclosure having a medium for trapping soot and other particulates, a bypass through the enclosure for diverting flow from the main flow path, heating elements embedded in the trapping medium, and a microprocessor control means for selectively regenerating the trapping medium in accordance with certain sensed conditions relating to the status of engine operation and the condition of a particulate trapping medium that is disposed in the main flow path. The main flow path has an electrostatic precipitator that is disposed in underlying relationship to the particulate trapping medium in the main flow path and a series of vanes are disposed in underlying relationship to the electrostatic precipitator and serve to redirect the exhaust flow passing through the main flow path and distribute the exhaust flow over the electrostatic precipitator and the trapping medium.

REFERENCE TO A RELATED APPLICATION

This application is a division of copending application Ser. No.265,547, filed Nov. 1, 1988, now U.S. Pat. No. 4,969,328, granted Nov.13, 1990; which was a continuation-in-part of my prior applications Ser.No. 921,330, filed Oct. 21, 1986 and now abandoned and Ser. No. 095,042,filed Sep. 9, 1987 and now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to a device for oxidizing certain exhaustemissions from diesel engines, for vehicles or stationary applications.

It is well known that diesel engines emit noxious exhaust by-productswhich are not only a nuisance to the public, but a health hazard aswell. Therefore, complete public acceptance of diesel engines will notoccur until the noxious constituents of their exhausts have beensubstantially cleaned up. The U.S. E.P.A. has established, byregulations, levels for such emissions which are at present consideredtolerable, but insofar as the applicant is aware, no one has been ableto design a system which can meet them satisfactorily.

This invention relates to a device containing a filter means forconnection to a diesel engine to process the exhaust. The devicecollects soot, and certain particulate emissions, and disposes of themduring engine operation either (1) by what is herein called "naturalregeneration" when the temperature of the filter is high enough, or else(2) by what is herein called "forced regeneration" when the filter, dueto insufficiently high temperatures, becomes loaded with particulates tothe pointwhere it either may not function properly or else otherwiseinterfere with the vehicle's operation. The latter type of regenerationis initiated by an associated electronic control unit which receivessignals from various sensing devices associated with the engine and/orwith the collecting and oxidizing device itself. In both types ofregeneration processes, soot which has been accumulated in the device isburned out and the by-product, carbon dioxide mainly, is disposed ofinto the atmosphere. The device may be endowed with particular catalystformulation which also oxidizes hydrocarbons and carbon monoxide therebysubstantially reducing, if not essentially entirely eliminating, dieselodor.

The invention comprises a number of features which individually andcollectively contribute to its ability to clean up diesel engineexhaust. These features relate to: (1) the sensing of when the devicehas become loaded to the point where forced regeneration should beinitiated; (2) the manner of by-passing the collection and oxidizingzone during forced regeneration; (3) control of the forced regenerationprocess; (4) the manner in which forced regeneration is performed; (5)the manner in which exhaust is forced to flow through the device; (6)the configuration of the filter; and (7) the overall organization of thedevice.

The foregoing features are disclosed in a first embodiment of theinvention. Also disclosed herein is a second embodiment which containsfurther features beneficial to soot collection efficiency. They are: 1)a pre-converter for the control of hydrocarbon (HC), carbon monoxide(CO), and volatile portion of the particulates; and 2) an electrostaticaugmented arrangement in which the particulates are pre-charged across acorona discharge medium ahead of the filter element.

The foregoing features, advantages, and benefits of the invention, alongwith additional ones, will be seen in the ensuing description and claimswhich should be considered in conjunction with the accompanyingdrawings. The drawings disclose a preferred embodiment of the inventionaccording to the best mode presently contemplated in carrying out theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a central longitudinal cross sectional view through a deviceembodying principles of the invention.

FIG. 2 is a fragmentary view showing additional detail.

FIG. 3 is a block diagram of the associated electronic control.

FIG. 4 is a partial transverse view through FIG. 1 on line 4--4.

FIG. 5 is a partial transverse view through FIG. 1 on line 5--5.

FIG. 6 is a partial transverse view through FIG. 1 on line 6--6.

FIG. 7 is a view similar to FIG. 1 but of a further embodimentcontaining additional features.

FIG. 8 is an enlarged axial cross section of an element of FIG. 7 byitself.

FIG. 9 is a transverse cross section on lines 9--9 in FIG. 7.

FIG. 10 is a transverse cross section on lines 10--10 in FIG. 7.

FIG. 11 is a central longitudinal cross sectional view through a secondembodiment of device embodying principles of the present invention.

FIG. 12 is a transverse cross sectional view taken in the direction ofarrows 12--12 in FIG. 11.

FIG. 13 is a transverse cross sectional view taken in the direction ofarrows 13--13 in FIG. 11.

FIG. 14 is a cross sectional view taken generally in the direction ofarrows 14--14 in FIG. 11.

FIGS. 15 and 16 are block diagrams illustrating microprocessor logicdiagrams that can be utilized in control of a diesel engine exhaustoxidizer.

FIG. 17 is an explanatory diagram.

DESCRIPTION OF THE PREFERRED EMBODIMENT I. General System Description

The overall system 10 comprises an oxidizer unit 12, an electroniccontrol unit 14, and various sensors to pick up input functions such astemperature, engine RPM, injection pump rack position, and backpressure.

A. Oxidizer Unit 12

The oxidizer unit 12 comprises outer and inner concentric shells 16, 18made of stainless steel and carbon steel construction to resist theharsh and high temperature environment. It has an inlet 20 whichconnects to the engine exhaust manifold outlet. The exhaust flow (arrow22) is directed either to a catalyst unit 24 within shell 16 or to aby-pass 26 which forms an annular space between the shells 16, 18.

A butterfly valve 28 is at the entrance to the two flow paths throughthe unit. The butterfly valve is operated through a vacuum actuator 30which, in turn, is operated by an electric solenoid switch 32 whichcontrols the vacuum signal 34 according to an electric output controlsignal received from electronic control unit 14. When the butterfly isin the position shown, flow is through the by-pass; when in the positionof the arrows 38, flow is through the catalyst unit 24.

A venturi 36 is installed ahead of the butterfly and by-pass entrance,thus creating a slightly lower pressure at the by-pass entrance when thebutterfly valve is in the open position.

The by-pass entrance 40 is normally closed by a spring-biased door 42;the door and mechanism 44 are so designed in relation to the venturi'seffect that when the butterfly is open to the filter unit 24, asillustrated, the pressure force on the by-pass door will be maintainedalmost constant under different engine RPMs and loads. The by-pass doorwill tend to lift open only when the back pressure due to soot build-upwithin the unit 24 increases to a threshold point.

The core of the unit is made of a series of guiding vanes 46a through46j specially designed to provide a uniform distribution of the flowover the surrounding catalyst 48. The series of vanes provides change inflow direction and redistribution which results in lowering flowvelocities substantially at the filter thus increasing the particulatecollection efficiency of the catalyst. The filter 24 is in the form ofan annular block made of super-imposed layers of stainless steel wiremesh as supporting material and stainless steel felt of fine strands.This combination provides adequate support while maintaining theefficiency of collection high. The two superimposed layers are rolled upto form the annular block. The flow direction through the catalyst unitis indicated by arrows 52.

Embedded in the catalyst is a heating unit 54 made of parallel elements56. The heating unit is used as an igniter to initiate forcedregeneration of the unit. In a way it functons similar to a match sothat the electric heat input required to initiate and maintainregeneration is low and within the practical load demand of theelectrical system of a diesel engine vehicle.

Installed at the outlet end of the annular space 58 surrounding thecatalyst block 48 is a thermocouple 60 that is used to monitor thetemperature and feed it to electronic control unit 14.

B. Electronic Control Unit (ECU) 14

The electronic control unit is made of various dedicated circuit boardsand logic elements. FIG. 3 illustrates the schematic of the unit.

C. Sensors 1. Injection pump rack position sensor 62

A standard magnetic pick-up 62 is used to transform the rack position(representing engine load) to one electric signal for electronic controlunit 14.

2. Engine RPM sensor 64

A standard magnetic pick-up is used to transform engine RPM to anotherelectric signal for electronic control unit 14.

3. Back-pressure switch 66

The mechanism 44 associated with by-pass door 42, in turn, operates aphoto-electric switch 66. This switch 66 transmits a signal toelectronic control unit 14 indicating a loaded trap.

4. Detail of ECU 14

ECU 14 comprises a frequency-to-voltage converter 14a which converts therpm pulses from sensor 64 into a corresponding voltage. This voltage,along with the rack position voltage from sensor 62, and the by-passvalve position voltage from photo-electric sensor 66, are inputs to acontrol logic circuit 14b. The control logic is constructed to performthe logical control functions herein described for commandingregeneration under the specified conditions. The control logic 14b hasits output connected to comparator and driver circuits 14c which drivethe butterfly valve in the manner described herein. There is also anamplifier 14d for amplifying the thermocouple signal and supplying itthrough circuits 14c for control in the manner described.

II. OPERATION OF SYSTEM

Under normal mode of operation no signal is forwarded to solenoid switch32 of the vacuum actuator, and butterfly valve 28 is in the normal openposition illustrated. The entire flow is directed toward the catalystwhere soot trapping takes place in addition to chemical reduction of HCand CO emission dependent on any catalyst which may be used on thefilter.

After few hours of operation soot accumulation reaches a point whereregeneration must take place to bring the trap back to originalcondition. Loading of the filter with soot is usually accompanied withincrease in back pressure and consequently performance and fuel losses.Unless exhaust temperature under some mode of operation reaches sootignition temperature and consequently the oxidizer regenerates in anatural mode, a forced regeneration is initiated by the electroniccontrol unit. This is the case always when the filter block is notcatalyzed since soot ignition temperature is always higher than exhausttemperature.

A. Forced Regeneration Mode

1. Forced regeneration is requested when back pressure continues tobuild up until threshhold value is reached, causing by-pass door 42 tobe popped open, and the photo-electric switch 66 sends a signal to theECU for the need for regeneration.

2. Regeneration will not actually start until favorable engine RPM,temperature, and load condition are reached. Usually, this requiresexhaust temperature sensed by thermocouple 60 to be above a certaintemperature, to reduce heater demand on the electric heating unit 54,engine RPM below a certain value, to reduce throttling effect throughthe by-pass, and engine load rack position below a certain value, toensure adequate supply of oxygen. If some of these conditions are notsatisfied during a certain time, such as the first 1/2 hour, after aregeneration request has been initiated, regeneration will start anyway.The driver of the vehicle has the option to start regeneration bypushing a switch 70 after an indicator light 72 showing a regenerationrequest has been turned on.

3. When forced regeneration starts, a second light 74 is lit and thefollowing steps take place:

(a) Butterfly valve 28 closes, and electric heater power is turned onfor a specified period of time.

(b) At the end of the above cycle, the power is cut off to the electricheater, and the butterfly valve will start duty cycling on and off everycertain number of seconds. The amount of valve closing time is afunction of the temperature at the thermocouple. Longer closing timeresults in lowering temperature due to oxygen starvation. A smallleakage in the butterfly valve is created to minimize smoldering of sootand keep slow combustion during prolonged valve closing. The valveopening time is such that the entire volume within catalyst is sweptwith fresh supply of exhaust (oxygen). This results in increasing thetemperature at the catalyst. The duration of valve opening time ismodulated by engine RPM. Duty cycling of the butterfly valve iscontrolled by the signal from the thermocouple, through closed loopoperation, and temperature is controlled at desirable level to ensureoptimum regeneration thus achieving complete combustion and burnout ofall soot accumulated. Also vital in the process is limiting the maximumtemperature to certain valve to avoid burnout and destruction of thecatalyst which is a universal problem with existing traps today.

(c) The process is terminated when the exhaust temperature continues todrop until a certain value is reached and valve opening frequencyincreases to a certain level indicating regeneration has been achieved.At this point in time, the ECU terminates the process and the butterflyvalve will return to the normal open position.

B. Natural Regeneration Mode

Natural regeneration, sometimes may be referred to as "continuousregeneration", is encountered when exhaust temperature is higher thansoot ignition temperature. This may be either a condition that resultsin continuous normal operation without undue effect on the filter, ordepending on trap load, it may result in the generation of excessiveheat and temperature which could be destructive to the filter. Thiscondition could arise in cold weather during start up due to high HC andCO emission followed by highway driving or due to engine problemswhereby oil consumption is too high.

Thermocouple 60 will sense the exhaust temperature and if thetemperature or rate of temperature rise with respect to time exceeds theset limits, the butterfly valve will duty cycle in an on-off modesimilar to the foregoing A-3 mode of operation. Through closed loopoperation, the temperature at the catalyst is controlled and a limit onthe maximum temperture is achieved. The process is terminated in amanner similar to the forced regeneration mode.

III. FEATURES A. System Design

The overall system design, which is a new breakthrough in the technologyof controlling diesel emission by exhaust after-treatment technique,will ensure that problems relating to filter burnout, fire hazards,catalyst durability and useful life are resolved satisfactorily in amanner acceptable to different applications of diesel engines invehicles or stationary applications. The double wall design will limitfire hazards to a minimum since high-temperatures during regeneration iscontained within the inner wall. All these features are contained in acompact design which is suitable for installation in the enginecompartment or under the floor in a vehicle.

B. Venturi, by-pass passive feature

The venturi by-pass design provides a passive concept for by-passing theflow. In case of butterfly valve, control unit, or loaded trapmalfunction, the by-pass feature will ensure that a flow area exists allthe time for the exhaust gases, and this unique design feature willensure that any malfunction in the active components of the oxidizersystem will not interfere with the continuous operation of the vehicle.The applicant acknowledges a drop in performance and some power losswhen this condition arises which is desirable to have repaired.

C. Venturi, by-pass as trap loading pickup

The venturi is designed to create a negative pressure at the by-passopening that is a function of flow speed. The pressure drop across thereactor (clean) creates a positive pressure at the by-pass door. Theventuri is designed so that the positive and negative pressure dropscompensate each other at all different speeds. The door at the by-passopening, therefore, will not open until the reactor is loaded withparticulate and thus an increase in the positive pressure, which isproportional to the degree of trap loading, will offset the spring loadon the door causing the door to open. Through the linkage mechanism, themotion is carried to the electrical switch 66 giving signal to the ECUthat the trap is loaded and needs regeneration. This arrangementprovides a positive, yet passive, feature of predicting trap loadingwhen compared with human use of pressure transducers and associatedproblems resulting from soot and moisture accumulation at dead spaces.

D. Regeneration Conrolled Temperature Through ECU

Regeneration controlled temperature through ECU and associated sensorsand actuators will result in what can be labeled an ideal regenerationthrough a well controlled process that has a minimum effect on emissionduring regeneration. The ECU will ensure that a design temperature isachieved and maintained constant during regeneration thus resulting inclean combusion of soots and reliable regeneration.

E. Embedded Electric Heater

Embedded electric heater within the catalyst, and the use of stainlesssteel felt will result in soot accumulation all around the electricelement and felt nearby. In initiating a forced regeneration, the heatenergy required to initiate and maintain a regeneration is a smallfraction of heat required by conventional systems in which heat isapplied ahead of the catalyst. This is in addition to lowering theheat-up time required to initiate soot ignition due to use of bare wirewithout sheathing around, thus reducing the thermal inertial effect ofthe wire (heating time for the wire is reduced to about 15 seconds). Inaddition, the use of elements without sheathing results in loweringthermal inertia and the time required for the element to reach ignitiontemperature (typically 15 seconds). The problem of maintaining lowthermal inertial and providing electric resistance between the elementand the steel felt is achieved by applying thin layer (0.005") ofceramic coating having major constituency of silicon oxide and bindingphases of silicates and phosphates. The location of the heating elementis such that maximum advantage of predominant modes of heat transfer(convection, radiation) are utilized to the maximum to achieve sootignition temperature all over the catalyst in the shortest period oftime.

F. Guide Vanes

The design of the vanes which is unique in this oxidizer will providethe following advantages: (1) minimum pressure drop, (2) equal flowdistribution all over the catalyst, which in turn leads to (3) radialflow through the catalyst with a linear velocity through the catalystthat is a fraction of the linear velocity at the inlet of the oxidizer(a ratio of 1:15 to 1:20). This is in comparison to known designs suchas axial flow or radial flow. A lower velocity in the catalyst has thefollowing advantages: (1) high filtration efficiency, (2) low Reynold'sNumber and low pressure drop, (3) minimum or low blowoff of particulatefrom the catalyst. This unique feature is achieved in the minimumpackaging volume which is almost a must in most applications either inthe engine compartment or under the floor.

F. 1. Calculation of Vane Sections

The vane's function is to segment, divert, and change flow directionfrom axial to radial. The entire cross section of the flow in the axialdirection is segmented into ten concentric segments for the ten vaneexample shown. Each concentric segment has an axial area equal toone-tenth the total axial flow area as shown in the diagram.Consequently, for a diameter D of the inlet 45, diameter D1 for 46a, D2for 46b, D3 for 46c, D4 for 46d, D5 for 46e, D6 for 46f, D7 for 46g, D8for 46h, and D9 for 46i, the following relationships exist: ##EQU1## Theuse of ten concentric segments is intended as an example. The actualnumber used in any given device will depend on certain factors, the mostsignificant of which is probably the size of the device. A smallerdesign could have fewer, such as five, or a larger design, more thanten.

G. Sandwiched Catalyst Design

This feature is intended to combine the advantage of wire mesh asstructurally stable and supportive material, but is slightly inferior asto collection efficiency and blowoff, and stainless steel felt which hashigh filtration efficiency and minimum blowoff, but unfortunately alsohas the possibility of plugging and is structurally unstable. Bysandwiching the felt between the wire mesh, it is possible to achievehigh filtration. Also, when the felt is loaded with particulate, thewire mesh behind it provides a collection room for soot dendrite thatwould break off the felt without causing plugging of the trap.

DESCRIPTION OF FIGS. 7-10 EMBODIMENT

FIGS. 7-10 show a further embodiment of the invention designated by thegeneral reference 200. It shares many common features with the FIG. 1embodiment, and they are identified by like reference numerals. The twomajor features of embodiment 200 which are not present in the FIG. 1embodiment are generally described as a pre-converter and anelectrostatic augumented converter.

Pre-converter

Particulate emissions generated from diesel engines are broadly dividedinto two portions: 1) solid carbon molecules; and 2) volatile portionscontaining hydrocarbon elements. Diesel particulates are defined as thedisperse matter collected on a filter at a temperature below 52 degreesC. (Ref.: 40 CFR, Part 86, Sub. B). Hydrocarbons having a dew pointabove 52 degrees C. will condense when cooled to 52 degrees C. or below.Particulates serve as condensation nuclei and the resultant particulateshave significant soluble organic fraction (SOF). SOF condensed mayaccount for 10 to 50% of total hydrocarbons, and constitutes between 1and 90% of the total particulate materials, dependent on engine type anduse of turbocharging.

The device of the present invention contains a trap converter andoperates at a temperature that is much higher than 52 degrees C. becauseit is located in the exhaust system. As such it exhibits rather verylittle catalytic activity at most. This is due to the non-use of noblemetals in the majority of cases and to the build-up of particulates onthe surface of the wire mesh which isolates the alumina-coated mesh fromthe gas stream. The net result is that the device (converter) will befunctioning in a mode that is being defined as a "total trap" for solidparticulates at the operating temperature of the trap. The volatileportion of the particulates, in gaseous form, will pass through the trapwithout any entrapment. The converter works in a mode as close to a"pure trap" for solid particulates.

Applications where the SOF is dominant or where higher filtration isneeded will require a pre-converter to work essentially on the gaseousportions of HC, CO, and SOF. A pre-converter functions essentially in amanner similar to a catalytic converter on a gasoline engine, but withsome differences.

A pre-converter in a diesel exhaust environment must be designed in sucha way as to maintain a clean surface with minimum build-up or entrapmentof particulates on the surface, while maintaining a good gaseousconversion efficiency. This requirement is essential since build-up ofsoot particulates on the surface of a catalyst could lead to burn-out ofthe accumulated soot under certain driving conditions (high temperaturefollowed by oxygen rich exhaust) leading to possible destruction of thecatalyst and/or burn-out of the pre-converter.

A pre-converter for a diesel control system is designed on the basis offlow velocity. Above a certain threshhold of flow velocity, particulatesthat have collected on the pre-converter are blown out and re-entrainedin the exhaust flow for capturing in the converter. In a vehicle, thedevice is designed to have this threshhold velocity occur at low tomoderate speeds, for example 25-30 miles per hour.

A diesel engine emission control system comprising a pre-converter and aconverter is capable of achieving high filtration efficiency for a broadnumber of diesel engine applications. The converter traps particulateswhile the pre-converter oxidizes gaseous and SOF pollutants. In FIG. 7the pre-converter is designated 202, and is disposed upstream of theconverter. For certain engines dependent on exhaust temperature profile,it may be preferable to place the pre-converter downstream of theconverter where it functions as a post-converter.

Electrostatic Augmented Converter

Conventional fibrous filters such as used in the converter often arelimited in performance and collection efficiency depending on the natureof particulates in the diesel exhaust.

An electrostatic augmented filter as shown in FIGS. 7-10 hasconsiderable improvement in performance, particularly higher collectionefficiency (especially for dry particulates normally generated fromturbo-charged diesels), and lower pressure drop across the fiber medium.

FIGS. 7-10 show an electrostatic precipitator 204 as an integral part ofthe converter. It consists of three coaxial perforated screens 206, 208,210, of successively larger diameter surrounding the vanes. The innerscreen is electrically grounded and joins to the perimeters of thevanes. The intermediate screen 208 is electrically insulated from therest of the converter and is connected to a high voltage terminal 211which in turn is connected to a high voltage D.C. power supply (notshown). Screen 208 is electrically charged from the power supply andwhen so charged, serves to pre-charge the radial flow of exhaust gaseswhich pass through its perforations. The high voltage is such that acorona discharge takes place between the screen 208 and the insidesurface of the screen 210. Although the screen 208 comprisesperforations in the form of circular holes formed by punching materialout of the screen, other forms of perforations may be used to advantage.For example, perforations formed by piercing rather than punching createelongated, cylindrical-walled perforations which have free edges towardscreen 210 at which the corona discharge occurs. This can result in fullionization of the particulate matter.

The screens 206, 208, 210 are maintained in spaced apart relation by aseries of high-voltage insulators 212 at appropriate locations. Sinceparticulates are electrically conductive, insulators are subject tomechanical failure caused by electrical sparking generated fromdisposition of conductive particulates on the surface. Arc-over resultsin localized overheating and the associated thermal stress leading tomechanical failures. Another mode of failure which results from arc-overis not necessarily accompanied by stress-induced mechanical failure.Rather, particle deposits may impregnate the insulator at the hightemperatures induced by arcing and ultimately lead to permanentshort-circuiting of the insulator.

The design of insulators in the device 200 is intended to yieldimprovements in performance and life expectancy. Details of an insulatorare seen in FIG. 8. Basically the insulator, which is of course of asuitable material, comprises a pair of disc-like heads 214, 216, whichare at the opposite ends of a cylindrical spacer 218. The insides of theheads comprise concentric circular grooves 220 which constitute air gapsforming stagnant spaces. The intention is to control the path of arcingsuch that arcing occurs mostly through the air, and in this way theresultant heat is dissipated mostly in the air rather than on theinsulator. The surface of the insulator is preferably glazed to minimizesurface adhesion of particulates.

The surface area of the insulator along which flow takes place is wheresoot tends to collect. The inclusion of the air gaps and stagnant spacesbreaks down the surface area such that soot collection is minimized,especially in the stagnant spaces. Soot accumulation in the stagnantspaces is very small in comparison to the surfaces more directly exposedto the flow. When arcing occurs, virtually all accumulated soot iscleared at and around the location where arcing occurs. At the air gapsthe soot is completely cleared. The arcing also results in substantialclearing of soot at other nearby locations on the insulator. Because theair gaps and stagnant spaces accumulate soot at much slower rates thanif the insulators did not possess such features, and because theyincrease the length of the electrical path, arcing frequency isdrastically reduced, and life expectancy is increased.

FIGS. 11-14 show a further embodiment of converter, or diesel emissionoxidizer, 300 which has a flatter profile than the precedingembodiments. A flatter profile may be an advantage in certainapplications, such as when the converter is mounted horizontally underthe floor of the body of a truck or car. The exhaust flow from a dieselengine enters the inlet 302. The exhaust flow is directed either to anelectrostatic precipitator/catalyst section 304 or to a bypass 306. Abutterfly valve 308 is at the entrance to the two flow paths through theconverter. When the butterfly is in the solid line position, flow is tothe section 304, but when the butterfly is in the broken line position,flow to section 304 is blocked. When the flow to section 304 is blockedby the butterfly, the entire flow passes through the bypass 306,entering the bypass at the perforations 305 in wall 307. A venturisection 310 is disposed just ahead of the butterfly.

The butterfly is opened and closed by a control (not shown in FIGS.11-14) which is the same as the control described for the firstembodiment. The embodiment of FIGS. 11-14 has no separate bypass doorsuch as the door 42 in FIG. 1. When the butterfly is in the solid lineposition, the venturi effect creates an acceleration in the flow whichwill result in the flow passing to the section 304, and not to thebypass. When the butterfly is in the solid line position, flow throughthe precipitator/catalyst is indicated by the arrows 314. When butterfly308 is in the broken line position, bypass flow through the converter isindicated by the reference numeral 312.

The precipitator/catalyst section 304 comprises a series of guide vanes316A, 316B, 316C that are arranged at the bottom to provide a generallyuniform upward distribution of the flow through theprecipitator/catalyst 304 in the manner indicated by the arrows 314. Thevanes provide a change in flow direction and a distribution whichresults in lowering the flow velocities, thereby increasing theparticulate collection efficiency of the catalyst.

The particular number of vanes that are employed will depend upon thespecific design and the three vanes shown in the drawing is an arbitrarynumber. The width, or lateral dimension, of the vanes is substantiallyequal to the width of the converter's enclosure.

The electrostatic precipitator portion of precipitator/catalyst 304overlies the vanes and comprises three generally rectangular shapedplates 318 which are spaced apart from each other in a generallyparallel manner. The middle plate 318 is supported from the enclosure bymeans of insulators 320. At least one of the insulators however isconstructed with provision to provide for an electrical connection tothe middle plate via an electrical terminal such as the terminal 322indicated in FIG. 14. The other two plates are joined to the enclosureand are therefore grounded. All plates 318 contain perforations toprovide for flow through the electrostatic precipitator portion. It isthe middle one of the three plates 318 that is connected via terminal322 to a high voltage DC power supply (not shown), and the electrostaticprecipitator operates to precharge the flow of exhaust gasses in thesame manner as described in connection with the electrostaticprecipitator of FIGS. 7-10.

The catalyst portion of precipitator/catalyst 304 comprises a stainlesssteel wire mesh 324. One or more ceramic coated electric heatingelements 326 passes through and is embedded in the wire mesh 324. Aceramic coating on the electric heating elements 326 provides for, amongother functions, electrical insulation between the heating elements andthe wire mesh when the heating elements are energized. The applicationof alumina washcoat to the mesh forms the mesh into a rigid matrix,similar to a cobweb, which has a significant void volume that may beapproximately 90%. As a result, contact points between the electricheating elements and the rigidly formed wire mesh are fixed in place forlife.

However, as a result of the heating elements' expansion and contraction,and associated raspe effect-friction, gas pulsation, vibration, aging,etc,-, some of the contact points between the mesh and the heatingelements may loose their ceramic coatings during the life of the device.This could in turn result in an electrical short circuiting at any suchpoint which would bypass the rest of the heating element as well asresulting in accelerated failure of the heated portion.

Tests that have been performed have shown that if a proper size isselected for the wire mesh that comes in contact with the heatingelements, the short circuit problem is self-correcting. Most of theheating currents are on the order of 50 to 80 amps. Stainless wireshaving hydraulic diameters in the range of 0.15 to 0.25 mm will burn outin 3 to 10 seconds should a short circuit develop, thereafter renderingthe short circuit open. Accordingly, in the event of certain ceramicdegradation, the provision of wire mesh strands that are below a certainsize will ensure self-correction of short circuiting. Since shortcircuiting will be a rather localized phenomenon wherever it occurs, arelatively minor ceramic degradation will not have any appreciableeffect on the performance of the device.

Converter 300 also comprises a dual purpose air injection system. Airinjection is a known procedure that has been used in particulate trapsfor regeneration. It provides adequate oxygen supply that enhances theprobability of a successful regeneration. Air injection is also a knownprocedure that has been used in blowing off aerosol particlesaccumulating around insulators in electrostatic precipitators, and assuch it prevents sparking and mechanical failure of the insulators.

Converter 300 is designed to regenerate under most driving conditionsexcept those associated with high acceleration, load and speed. In mostautomotive applications sudden change in driving patterns can increasethe probability of incomplete regeneration. Although an incompleteregeneration is detected and accounted for and has no impact onoperability or emission, the probability of incomplete regenerations maywarrant the use of air injection.

Accordingly, in converter 300 injected air from a suitable air supply,such as a pump (not shown) for example, is introduced into section 304.Within converter 300, tubes 332 convey the injected air to theindividual insulators. This injected air is used in conjunction with theexhaust flow for the combustion of soot during regeneration and also formaintaining the insulators in a reasonably clean condition.

A further aspect of the invention relates to the reduction of peaktemperatures leaving the outlet of the converter during regeneration.The regeneration process is controlled through duty cycling of thebutterfly valve in the manner described for FIG. 1. This results in therepeated diverting of the exhaust flow into two segments, namely a flowsegment that has passed through precipitator/catalyst 304 followed by aflow segment that has passed through the bypass. Each of these twosegments typically has a duration of approximately one to three seconds.Since these segments discharge at the downstream outlet end of theconverter, it is possible for segments of high temperature gases toescape from the tailpipe at temperatures close to the regenerationtemperature. Such high temperatures, which could reach 1400 degrees F.,may pose a fire/safety hazard around the vehicle, particularly at lowtraffic speeds or in congested areas. This problem is likely to beencountered on some applications where the converter is installed awayfrom the engine and the tailpipe section that is downstream from theconverter is short.

This problem can be resolved in a practical fashion by installing athermal stabilizer downstream from the converter in the outlet 335.FIGS. 11 and 14 show such a thermal stabilizer 334 disposed in theoutlet section of the converter. The stabilizer is an insert similar inconstruction to a preconverter but without catalysts. The insert has asufficient surface area to enhance thermal conductivity between the hotgases and the insert itself, and it should also have sufficient thermalinertia (weight) to absorb the required amount of heat from a hot gassegment, or column, as that hot gas segment passes through the thermalstabilizer. The heat that is absorbed by the thermal stabilizer isexchanged back to the bypass flow segment, or column, when the coolerbypass flow segment passes through. This process of extracting heat fromthe flow that has passed through the precipitator/catalyst and thenrejecting heat to flow that has passed through the bypass occurs duringeach cycle of butterfly valve operation. In this way, the thermalstabilizer reduces the peak exhaust gas temperatures at the tailpipeduring regeneration. Where the precipitator/catalyst flow gases may beas much as 1400 degrees F. and the bypass flow gases in the range of 300to 600 degrees F., the thermal stabilizer can reduce the maximumtailpipe temperature to an acceptable level.

While it is expected that a converter system will require maintenance atcertain maintenance intervals, the system will not regenerate if it isnot maintained and a malfunction develops in any of the activecomponents except for the butterfly valve. Converter 300 is designedsuch that the pressure drop through a completely soot-laden wire mesh islower than the pressure drop required to force the exhaust flow throughthe bypass. If the system is not able to regenerate, it automaticallydefaults to an agglorometer mode of operation so long as thespring-loaded butterfly valve 308 assumes the solid line (open)position. In this mode of operation, all soot coming into the wire meshis collected and then subsequently blown off in the form of coarsedenderites. Accordingly, the problems associated with particulateemissions entrained in exhaust gases, namely, air pollution, are keptunder control; however, the soiling from denderites will remain sincethese denderites will pass out of the tailpipe and fall on the ground.It is believed that the emission of dendrites is preferable toexhaust-entrained particulates and therefore, the automatic shifting toan agglomerator mode of operation in the event of a failure of any ofthe active components is deemed desirable.

One of the ways to signal trap loading is to utilize differentialpressure transducers before and after the converter and to develop asignal when a predetermined amount of trap loading is sensed. Thissignal is used to initiate regeneration. Since the transducers areexposed to soot and moisture accumulation, the transducers maymalfunction. Moreover, the use of transducers adds to the overallconverter cost.

An aspect of the converter that is disclosed herein relates to a methodfor predicting soot emission based on engine soot mapping data developedfrom actual test data from actual engines. Consequently the use ofpressure transducers to sense trap loading can be eliminated. For agiven engine, the amount of soot generated over time by the engine maybe approximated by integrating engine RPM (speed) and rack position(load) with respect to time and then comparing this result with knownsoot mapping data for the particular model of engine. The process mayeven be refined by incorporating a correction factor which is equivalentto an emission deterioration factor to account for the increase inemission with accumulated mileage. The engine RPM and rack position aresensed by electronic sensors already on the engine. A microcomputer isutilized to perform the integration functions and the comparison withengine soot mapping data.

It is to be appreciated that this method may not precisely predict theactual soot accumulation and hence, it may not be suitable for certaintypes of traps such as ceramic traps, since the margin of error isbeyond the allowable regeneration zone.

In a converter of the type represented by converter 300 characterized byhigh soot retention capacity for up to 10 or more hours of operationbefore regeneration is needed, the regeneration allowable operating zoneis bounded on one end by a threshhold of incomplete regeneration and onthe other end by a threshhold of blow-off. This zone is substantiallywide enough to accommodate the variance in predicting trap loadingthrough use of the integration of engine RPM and rack position withrespect to time and comparison thereof with engine soot mapping data.FIG. 17 is a diagram showing the regeneration allowable operating zonefor both converter 300 and for a typical ceramic trap. The ceramic trapmust be regenerated much more frequently and within a smaller allowablezone than converter 300.

FIG. 17 shows two curves a and b. Curve a is symmetric about the designloading, and represents the probability of the accumulated sootmeasurement being high. Curve b is skewed to the right, and representsthe probability of the accumulated soot measurement being high whiletaking into account systems delays which may occur before actualregeneration starts.

FIG. 15 portrays a logic flow diagram for the operation of amicroprocessor that is utilized for control of the regeneration processin converter 300 when the converter is used in a transit busapplication. The first step in the process is to compile the level ofsoot accumulation in the particulate trap by integrating the rackposition and RPM as a function of time in the manner previouslydescribed. So long as the soot accumulation does not reach apredetermined high level, this compilation continues. When the sootreaches a first predetermined high level, a level 1 warning starts. Theprobability of arriving at the beginning of a level 1 warning isrepresented by curve a in FIG. 17. This first warning level is a visualone that is provided by a warning light on the dash panel of thevehicle. If the converter is not regenerated within a certain amount oftime, the second level of warning is initiated and this is preferably anaudible sound that is given in conjunction with the first warning. Theprobability of starting regeneration during either a level 1 or a level2 warning is represented by curve b in FIG. 17. Regeneration may bestarted either automatically or manually. As explained earlier, thisdepends upon the particular driving conditions. In the case of a transitbus, it is unlikely that city revenue service will be conducive toregeneration. Therefore regeneration will probably have to be manuallyinitiated when the vehicle has returned to its yard and is idling.

Regeneration initially involves energizing the electric heating elements326 for a preset time, closing butterfly valve 308 for a preset time,and thereafter duty-cycling the butterfly. The electric heater elementsare turned off after a certain amount of energization. Duringregeneration, temperature data is compiled to determine whether asuccessful regeneration has occurred. The regeneration temperature ismonitored with respect to time to provide an indication of the thermalheating units released during regeneration. If the monitored valueequals or exceeds a reference value, regeneration is considered completein which case the process of compiling soot accumulation in the newcycle can begin anew. That portion of the logic diagram between theblock entitled "Close Butterfly Valve For Preset Time" and the block"Terminate Duty Cycle Open Butterfly" (String B of the diagram) involvesthe duty cycling of the butterfly valve in accordance with the techniquedescribed earlier for FIG. 1. It is during the execution of this portionof the cycle that the temperature data is being compiled.

As stated above, a successful regeneration cycle results in beginning tocompile soot accumulation information from a starting level in which theinitial soot accumulation is considered to be zero. In the event,however, that the compiled temperature data during regenerationindicated that a successful regeneration had not occurred, then it mustbe assumed that there is a certain amount of unburned soot left in thetrap. In such a case, the compilation of soot from the new cycle is ineffect, but it begins at a point at which it is assumed that a certainamount of soot exists in the trap due to the fact that not all the sootwas burned off during the unsuccessfully completed regeneration. Becauseof this crediting of unburned soot into the soot accumulation, the nextregeneration cycle will start earlier than it otherwise would if therewere no credit for the unburned soot. This procedure ensures againstencountering blow off in the new cycle.

In certain applications, such as transit bus application, the drivingpatterns are not conducive to the probability of a successfulregeneration while the vehicle is being operated on the road. For thistype of application the converter can be designed with a high retentioncapacity to operate for an extended period of time without regeneration.In this type of a situation, regeneration can be initiated manually byoperating the transit bus at idle speed once it has returned to itsservice facility or service yard.

FIG. 16 illustrates a microprocessor logic diagram for a convertercontrol that is used in truck and automotive type application asdistinguished from transit bus type applications. The strings A and B ofthe logic diagram of FIG. 16 are identical to those of the strings A andB of the logic diagram of FIG. 15 except that in the logic diagram ofFIG. 16, regeneration is started automatically. A further similarity inboth logic diagrams is that the soot accumulation is compiled byintegrating rack position and engine speed with respect to time. Whenthe soot accumulation has reached a predetermined high level which isindicative of a need for regeneration (curve a in FIG. 17), the controlthen determines whether other conditions are conducive to initiating aregeneration. These conditions are the temperature of the exhaust, theengine load, and the engine speed. So long as the exhaust temperature issufficiently high and the engine load and speed are not too excessive toensure adequate oxygen content, then regeneration automatically takesplace (curve b in FIG. 17). In other words, regeneration automaticallyoccurs so long as the vehicle is being operated under proper drivingconditions. Such proper driving conditions would be those conditionswhere the engine speed is not excessive and where the engine load is notexcessive and where the exhaust temperature is sufficiently high toenhance combustion of the soot. It may happen that proper drivingconditions are not reached within the nominal design range of theconverter, but a delayed regeneration can still be within theregeneration allowable operating zone. In other words, the typicaloperation of an automobile or truck is such that there is an extremelyhigh probability that conditions conducive to regeneration will occurwithin the regeneration allowable operating zone. In the unlikely eventof having regeneration beyond the boundaries of the allowable operatingzone, no adverse effects are encountered and the deviation will becorrected in the next regeneration cycle. The reason why the abovedisclosed technique of determining trap loading without transducers canbe used is because the soot retaining capacity of the present convertersis significantly high when compared to ceramic traps. Hence, althoughthe prediction of trap loading by integrating engine speed and rackposition with time may not be as accurate as when using pressuredifferential transducers, the error margin is well within theregeneration allowable operating zone.

As noted earlier, the converter is designed such that the pressure dropthrough the wire mesh when completely laden with soot is lower than thepressure drop required to force the flow through the bypass. Hence, whenthe butterfly is in the solid line position, there is no flow from inlet302 into the bypass. To the contrary, with a clean trap and even up tothe point where the trap becomes completely laden with soot, the flowthrough section 304 will actually result in a small recirculationthrough the bypass meaning a small reverse flow through the bypass in adirection opposite arrow 312. The amount of recirculation, of course,progressively decreases as the trap loading increases. With thisorganization and arrangement then, the bypass door 42 that was presentin the version shown in FIG. 1 is unnecessary in converter 300.

I claim:
 1. A diesel engine exhaust oxidizing device comprising anenclosure having an inlet for receiving diesel engine exhaust, a mainflow path through said enclosure to an outlet of the enclosure, saidmain flow path containing a medium for trapping particulate materialsuch as soot and removing significant amounts thereof from the exhaustflow through said main flow path, a by-pass through said enclosure fordiverting flow from said main flow path, and a microprocessor controlmeans for selectively controlling the flow through said main flow pathand said by-pass in accordance with certain sensed conditions relatingto the status of engine operation, in which the medium is annular inshape and including a series of nested annular vanes along the length ofthe interior of the annular medium for taking axial flow and directingit radially outwardly through the medium.
 2. A diesel engine exhaustoxidizing device as set forth in claim 1 in which the medium comprisessuperimposed layers of wire mesh and wire felt rolled into an annularform.
 3. A device as set forth in claim 1 including a converter in linewith said device for oxidizing gaseous constituents of the exhaust.
 4. Adevice as set forth in claim 1 including an electrostatic precipitatorthrough which radial flow from the vanes is passed before reaching saidmedium.
 5. A device as set forth in claim 4 in which said electrostaticprecipitator comprises coaxial cylindrical screens which are separatedby insulators which comprise air gaps forming stagnant spaces.
 6. Adiesel engine exhaust oxidizing device comprising an enclosure having aninlet for receiving diesel engine exhaust, a main flow path through saidenclosure to an outlet of the enclosure, said main flow path containinga medium for trapping particulate material such as soot and removingsignificant amounts thereof from the exhaust flow through said main flowpath, a by-pass through said enclosure for diverting flow from said mainflow path, and a microprocessor control means for selectivelycontrolling the flow through said main flow path and said by-pass inaccordance with certain sensed conditions relating to the status ofengine operation, said control means comprises a microprocessor fordetermining whether engine conditions are conducive to initiatingregeneration of said trapping medium and a valve means for selectivelyopening and closing said main flow path and said by-pass to exhaust flowwhich enters the enclosure at said inlet, said enclosure comprises adouble-walled cylinder, said main flow path being through an interiorportion and the by-pass being through a surrounding portion surroundingsaid interior portion, means for regenerating the medium after a certainamount of particulate collection by the medium, means for opening saidby-pass to flow during regeneration of the medium, and means to entrainthe regeneration product with the flow through the by-pass prior todischarge of the flow through said outlet of said enclosure.
 7. A dieselengine exhaust oxidizing device as set forth in claim 6 in which themedium comprises superimposed layers of wire mesh and wire felt rolledinto an annular form.
 8. A diesel engine exhaust oxidizing device as setforth in claim 7 including heating elements passing axially through saidmedium.
 9. A diesel engine exhaust oxidizing device as set forth inclaim 8 in which said heating elements are activated below a certainsensed temperature when said mechanism is open.
 10. A diesel engineexhaust oxidizing device as set forth in claim 8 in which said heatingelements comprise ceramic coated wire.
 11. A diesel engine exhaustoxidizing device as set forth in claim 6 in which said microprocessorcontrol means includes a thermocouple for sensing temperature of exhaustproducts and an ECU connected to the thermocouple for maintaining theregeneration temperature within a substantially constant range whichcombusts the trapped particulates and soot without damaging said medium.12. A diesel engine exhaust oxidizing device as set forth in claim 6including means for causing regeneration action to be taken by means foradding a certain heat input to the medium by an electric operated heaterand then cycling the butterfly valve open and closed after the electricoperated heater has been shut off.
 13. A device as set forth in claim 6wherein said trapping medium is annular in shape, and said flow pathincludes a means for directing the exhaust flow radially through themedium, and an annular shaped electrostatic precipitator through whichthe radial exhaust flow is caused to pass before reaching said medium.14. A device as set forth in claim 13 in which said electrostaticprecipitator comprises coaxial cylindrical screens which are separatedby insulators which comprise air gaps forming stagnant spaces.
 15. Adevice as set forth in claim 6 further comprising a thermal stabilizerdisposed in said outlet for attenuating the peak temperature of gasesdischarged from the device by acting as a heat sink to the relativelyhotter segments of gases and acting as a heat source to the relativelycooler segments of gases.
 16. A device as set forth in claim 6 furthercomprising an electric heating element that is embedded in said trappingmedium, said electric heating element comprising a ceramic coating andsaid medium comprising wire mesh strands that come in direct contactwith the ceramic coated heating element, said wire mesh strands beingselected to have a mean hydraulic diameter that will cause them to burnout in a relatively short period of time in the event of failure of theceramic coating at any point where any of the wire mesh strands are incontact with the heating element whereby a short circuiting of theheating element due to degradation in the ceramic coating on the heatingelement that gives rise to short circuiting of the heating elementthrough wire strands that are in contact with the heating element at thepoint of ceramic degradation will be self-correcting because the strandsthat are in contact with the heating element at the point of ceramicdegradation will burn out thereby rendering the short circuit an opencircuit.
 17. A device as set forth in claim 14 further comprising an airinjection means for injecting air into the enclosure and directing theair over the insulators to keep them clean and also improving combustionof the collected soot in said medium during regeneration of said medium.